Footwear typically comprises an outsole bonded or stitched to an upper. Inside the footwear is usually a rigid insole shaped so as to conform as closely as possible to the sole of a user's foot, covered by a sockliner made from a flexible fabric, which increases the user's comfort and the durability of the footwear.
The comfort of the user is largely dependent upon how snugly the footwear fits the user's foot. This in turn depends upon the degree to which the insole conforms to the shape of the user's foot and the size of the space within the upper, which determines how readily the foot can shift positions within the footwear. For an optimum fit, the insole should not only compliment the topography of the sole of the foot, but must also support the periphery of the foot to prevent lateral shifting (and attendant abrasion) within the footwear, and the upper must comfortably maintain the foot in a relatively stable position on the insole. There are many shapes and sizes of feet, and manufacturers accordingly produce footwear in different sizes to accommodate a range of foot sizes and shapes so that, in general, an adult or child can find footwear which will closely and comfortably conform to the shape and size of his or her feet.
However, fitting footwear to children poses a unique problem, because of the rate at which their feet grow. It is generally impractical to purchase footwear that comfortably fits a child's feet at the time of purchase, because the child will quickly outgrow the footwear. Preferring to have their children wear footwear that is too large, rather than footwear that is too small and thus constricts or abrades against the child's feet, parents typically purchase footwear which is too large for the child's feet anticipating that child will grow into the footwear in a short time. In result, children frequently spend prolonged periods wearing footwear that is too large to fit comfortably. This can result in chronic discomfort, and in extreme cases more serious foot ailments.
Moreover, as quickly as a child grows into the footwear he or she will grow out of it, and new footwear must be purchased. Thus, while adults will usually wear out footwear before discarding it, a child's footwear tends to outlast the ability of the footwear to fit the child, so that otherwise usable footwear ends up being discarded or going into disuse.